The histamine receptors, H1, H2, H3 and H4 are well-identified forms. The H1 receptors are those that mediate the response antagonized by conventional antihistamines. H1 receptors are present, for example, in the ileum, the skin, and the bronchial smooth muscle of humans and other mammals. Through H2 receptor-mediated responses, histamine stimulates gastric acid secretion in mammals and the chronotropic effect in isolated mammalian atria. H4 receptors are expressed primarily on eosinophils and mast cells and have been shown to be involved in the chemotaxis of both cell types.
H3 receptor sites are found on sympathetic nerves, where they modulate sympathetic neurotransmission and attenuate a variety of end organ responses under control of the sympathetic nervous system. Specifically, H3 receptor activation by histamine attenuates nonepinephrine outflow to resistance and capacitance vessels, causing vasodilation.
Imidazole H3 receptor antagonists are well known in the art. More recently, non-imidazole H3 receptor antagonists have been disclosed in U.S. Pat. No. 6,720,328, and in US Published Applications 2003/0109564, 2004/0097483, 2004/0048843 and 2004/0019099.
U.S. Pat. No. 5,869,479 discloses compositions for the treatment of the symptoms of allergic rhinitis using a combination of at least one histamine H1 receptor antagonist and at least one histamine H3 receptor antagonist.